A Klebsiella pneumoniae isolate from urine shows a cefotaxime zone of 22 mm, suggestive of possible ESBL production. Which phenotypic method is recommended to confirm ESBL production in this isolate?
- A Testing cefotaxime-clavulanate combination disc and looking for a zone enhancement of 5 mm or more over cefotaxime alone ✓
- B Comparing zones of cefotaxime and ceftazidime discs alone placed 30 mm apart
- C Repeating the disc test after pre-incubating the plate at 44 degrees Celsius
- D Measuring the minimum bactericidal concentration of cefotaxime in broth
Correct answer: A. Testing cefotaxime-clavulanate combination disc and looking for a zone enhancement of 5 mm or more over cefotaxime alone
Explanation
ESBLs are inhibited by clavulanic acid, so the standard phenotypic confirmation places a cephalosporin disc with and without clavulanate on the same lawn. An increase of 5 mm or more in the inhibition zone around the combination disc compared with the cephalosporin alone confirms ESBL production. Simply comparing different cephalosporins without an inhibitor cannot distinguish ESBL from other resistance mechanisms, and MBC determination does not identify the enzymatic basis of resistance.
Reference: Mackie and McCartney Practical Medical Microbiology, 14th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.